Skew compensation for raster image transfer device

ABSTRACT

A multifunction image-transfer system with printing, scanning, digital copying, and facsimile capabilities detects skew as sheet media is fed. A skew processor digitally skews image data as a function of sheet-feed skew detected by skew detectors. In the context of a scanning operation, an image-bearing document is fed to a scanner device, which generates a digital raster image. A buffer stores portions of the digital image as it is transferred. The image data in the buffer is digital skew-compensated before it is transferred to a host computer. In the context of a printing operation, raster print data is stored in the memory buffer and then skew compensated before the image is printed to the print media. The digital skew compensation allows mechanical tolerances in such systems to be relaxed and system lifetimes to be lengthened since “true” images can result despite moderate media skew.

BACKGROUND OF THE INVENTION

The present invention relates to image-transfer devices and, moreparticularly, to devices, such as scanners and printers, that transferimages between digital and hard-copy formats.

Much of modern progress is associated with the increasing prevalence ofcomputers that manipulate data in digital form. Peripherals such asscanners and printers provide important links between the human andcomputer realms. In addition, related technologies, such as facsimile(fax) machines and digital copiers are becoming evermore prevalent.

Most image-transfer devices, excluding some flat-bed scanners, requiresome form of sheet-feed device, either for handling documents to bescanned or for handling blank media to which an image is to betransferred. (As used herein, “blank media” includes all media to whichimages are to be added, even media with pre-existing images, such asletterhead stationery.) Many scanners use automated document feeders.Printers typically include a blank-media sheet feeder. In fact, manyprinters have mechanisms that support two or more media sources. Devicesthat support both printing and scanning, such as fax machines andmultifunction office machines, typically use both document feeders andblank-media feeders.

As the costs of electronic and optical components are falling,sheet-feed mechanisms are consuming increasing proportions of the costsof image-transfer devices. The market expectation is that thesheet-feeding mechanisms draw media straight (i.e., “true”) through theimaging transfer section of a device. In many cases, the true media feedis to be accomplished with a wide variety of sheet media, includingpaper of different weights, card stock, transparency film, andphoto-paper. Moreover, sheet feeding must be performed to specificationsover thousands of sheets.

Deviations from straight paper feeding can result in image skew, i.e.,the image is tilted relative to the media. In the case of scanning, thedigital image is tilted relative to the sheet-media source. In the caseof printing, the sheet-media image is tilted relative to the digitalimage source. In general, perceptible skew is highly undesirable,although unperceptible skew may be tolerable for some applications.However, manufacturing a sheet-feed mechanism that maintains skew withinacceptable tolerances for different media over thousands of feeds iscostly.

There is scanning software that can be used to correct for skew in animage after it has been completely transferred to a host computer. Suchsoftware makes assumptions about what an image should look like, e.g.,the software assumes lines should be either horizontal or vertical,rather than oblique; such software may not be effective for images thatdo not conform to those assumptions. Furthermore, such software onlyapplies to scanning, not to printing. What is needed is a moreeconomical method for maintaining printed and scanned image skew withinacceptable tolerances regardless of the type of image.

SUMMARY OF THE INVENTION

The present invention provides an image-transfer device and method fordetecting mechanical skew of sheet media fed thereby and skewing digitalimage data as a function of the detected sheet-feed skew to compensatefor the sheet feed skew. The digital image data ad can be acquired fromthe media (e.g., in the case of a scanner) or it can be to be applied tothe media (e.g., in the case of a printer). The invention thus helpsprovide for “true” images despite skewed media feeds. In one embodiment,the invention comprises a sheet-feed mechanism, a media-skew detector, adigital memory for storing image data, and a controller. The storedimage data corresponds to an image that may require skew compensation.The controller generates a skew function from the media-skewdetermination and then applies the skew function to the uncompensatedimage data to yield skewed image data. The invention may also providefor skew functions that achieve objectives, such as image scaling, inaddition to image skewing.

The invention does not require that the entire image to be skewed berepresented in memory at one time. Each pixel in the skew-compensatedimage can be characterized as a function of a small number ofneighboring pixels in the uncompensated image. Thus, only a fraction,less than half, of the data for the uncompensated image need beavailable at any one time. Thus, the memory can be a buffer that storesdata corresponding to some, but not all of the raster lines of theuncompensated image.

The invention provides an economical data-processing approach tocompensating for skewed media feeds. In the case of a print operation,an image may be skew-compensated in the digital domain before it isapplied to media. In the case of a scan operation, the image may beskew-compensated in the digital domain after it is acquired from adocument. The skew compensation in the digital domain allows mechanicalsheet-feed tolerances to be relaxed. In addition, device lifetimes canbe extended, as degradation in sheet-media handling over time can becompensated electronically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a multifunction imaging systemincluding scanning and printing functions in accordance with oneembodiment of the present invention.

FIG. 2 is a flow chart for a method of one embodiment of the inventionpracticed in the context of the system of FIG. 1.

FIG. 3 is a flow diagram for print function of the imaging system ofFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a multifunction imaging systemAP1 provides for print, scan, digital copy, and facsimile functions, asshown in FIG. 1. The print function operates in response to commands andprint data received from a host computer 10. The scan function can beinitiated by host computer 10 or locally, in which case system AP1signals host computer 10 that it is to be receiving scan data. Digitalcopying involves scanning and then printing and can be handled on astandalone basis.

Faxing, which allows graphic communications with a remote facsimilemachine 12, has both transmit and receive modes. Transmit mode issimilar to the scan function, while receive mode is similar to the printfunction. To enable faxing, imaging system AP1 is plugged into atelecommunications outlet 14, which is coupled via a telecommunicationssystem 16 to another telecommunications outlet 18 into which remotefacsimile 12 machine is plugged.

Imaging system AP1 provides a print-media path 20, a scan-media path 30,and a data path 40. Print-media path 20 inlcudes a blank-media sheetfeeder 21, a skew detector 23, and a laser-image transfer device 25, theoutput of which is print media bearing a printed image. Scan-media path30 includes a document feeder 31, a skew detector 33, and a scanningdevice 35, the output of which is rasterized image data. Data path 40includes a host-computer interface 41 for interfacing with a hostcomputer 10, a rasterizer 43, a multi-line buffer 45, and a controller47. Controller 47 includes a re-writable skew table 51 and a skewprocessor 53 used for “skewing” digital-image data. Multi-line buffer 45is a dual-ported RAM with an address port AD, a data input DI, and adata output DQ.

A typical print operation can begin with host computer 10 sending printcommands and data to imaging system AP1, where they are received byrasterizer 43 via interface 41. Rasterizer 43 performs a variety ofprint-control functions, including rasterizing vector image data into araster image format. Rasterizer 43 also can convert between somestandard raster formats and its internal raster format. If rasterizer 43receives data in its internal raster format, it can simply store thatraster data. Rasterizer 43 has sufficient memory to store1-bit-per-pixel (as defined by the scan and print resolutions) for afull-sized (e.g., letter-sized or A4-sized) image. Rasterizer 43 canalso parse print-control signals and forward them to controller 47.

In response to receiving a print-control signal from rasterizer 43,controller 47 commands blank-media sheet feeder 21 to feed a sheet ofblank media 61. Blank-media feeder 21 includes sheet-media guides 63 and65, the latter being movable to match the width of the media being fed.The media width as determined by the setting of sheet-media guide 65 iscommunicated to controller 47. Guides 63 and 65 help limit, but do notnecessarily eliminate, skewing of media 61 as it is fed.

As blank media 61 is fed, it passes skew detector 23. Skew detector 23has two edge detectors 67 and 69. Each edge detector 67, 69 includes alight-emitting diode 71, a photo-diode (light-detecting diode) 73, and alever 75. When a sheet begins to feed, light-emitting diodes 71 areactivated and photo-diodes 73 are monitored by controller 47. Before asheet arrives, photo-diodes 71 detect light from their respectivelight-emitting diodes 73. As the leading end of a sheet is fed, itsweight depresses each lever 75 so that it pivots into position betweenthe respective light-emitting diode 71 and the respective photo-diode73, breaking (occluding) the optical link. The resulting high-to-lowdetection signal transitions are detected by controller 47.

Edge detector 69 is mechanically coupled to the movable sheet-mediaguide 65 so that edge detectors 67 and 69 are positioned about onecentimeter (1 cm) from each lateral edge of the sheet being fed. Theposition of movable sheet-media guide 65 is communicated to controller47, so that the width of the media is known. This width is used alongwith other information in generated skew table 51.

If the sheet feed is true, edge detectors 67 and 69 indicate occlusionsimultaneously. In this case, there is no skew and thus no need for skewcompensation. If one of the edge detectors 67, 69 indicates occlusionbefore the other, then there is skew. Since the media is fed at a knownrate, the time between occlusion of one edge detector and the otherindicates a skew length, the distance by which one lateral edge of themedia 61 leads the other. Controller 47 records which edge detector isoccluded first to determine the direction of skew. Controller 47 alsotimes the interval between occlusions to determine the length associatedwith the skew. This length is divided by the known media width (asdetermined from the known position of movable media guide 65) todetermine a skew angle.

Controller 47 uses the determined skew angle to generate the contents ofskew table 51. Skew table 51 indicates what function is to be used ingenerating a skew-compensated image. In general, the function can be aweighted average of color values or toner amounts associated with onepixel for one or more pixels in an uncompensated digital image. Anexemplary skew-compensation function is discussed further below withreference to FIG. 3.

Concurrent with the feeding of blank media 61, rasterizer 43 beginsoutputting raster image data to multi-line buffer 45. In otherembodiments, the data is not reformatted as it is sent to a buffer. Inthis case, 1-bit-per-pixel data is converted to 7-bit per pixel data toeffect resolution enhancement. This conversion is performed“on-the-fly”, obviating the need for a memory large enough to store afull-sized sheet's worth of 7-bit-per-pixel image data. Instead, afraction at a time of the 7-bit data is stored in multi-line buffer 45.

Multi-line buffer 45 is a dual-port random-access-memory functioning asa 120-row and 4800-column table of 8-bit storage locations (seven bitsof each storage location are used during a print opreation). The 4800columns correspond to eight-inch wide media (letter size with ¼″margins) at 600 dots-per-inch (dpi). The 120 rows corresponds to 0.2″ at600 dpi in the direction of media travel. The 0.2″ corresponds to themaximum skew that can be digitally corrected in system AP1.

Digital-skew processor 53 transmits skew-compensated raster image datato laser printer device 25. This raster image data has the same 7-bitformat as the data in buffer 45. However, the image represented by theoutput of processor 53 is skewed to the extent indicated by skewdetector 23. The skew-compensation function is applied solely to datastored in multi-line buffer 45 at any given time (as opposed to otherimage data not presently stored in buffer 45).

The skew-compensated data is received by laser-printer device 25. Thedata is converted to pulse-width modulation signals, which are used todrive a laser. The laser, in turn, determines the pattern of toner to beapplied to the sheet-media 61 being fed through device 25. The result isan image-bearing document 61′ in which the printed image is aligned withrespect to the media as it would be in the absence of sheet-feed skewand without digital skew compensation.

A scanning operation can begin with inserting an image-bearing document81 (the image can be text) in document feeder 31 and having hostcomputer 10 transmit a scan command to imaging system AP1. This commandis forwarded by interface 41 and rasterizer 43 to controller 47, whichtransmits it to document feeder 31. This command initiates feeding ofthe image-bearing document 81 so that it is detected by skew detector33. Skew is limited by document-feed guides 83 and 85. Document-feedguide 85 can be moved so that the paper guide path matches width ofdocument 81.

Skew detector 33, essentially identical to skew detector 23, includesedge detectors 87 and 89. Controller 47 determines skew length from theskew detection signals provided by skew detector 33. Skew angle isdetermined by dividing skew length by the document width indicated bymovable document-feed guide 85, to which edge detector 89 ismechanically coupled. Scanner device 35 provides 8-bit-per-pixelraster-image data to buffer 45. Skew processor 53 skew-compensates thedata in buffer 45 to yield 8-bit-per-pixel skew-compensated data. Thisdata is transmitted through rasterizer 43 and interface 41 to hostcomputer 10 for use by application programs running thereon.

Note that a fax-receive operation is essentially the same as a printoperation except that the data and control signals are received fromremote fax 12. Likewise, a fax-transmit operation is essentially thesame as a scan operation, except the scanned data is formatted accordingto fax standards after skew-compensation. The fax data is then sent tofax machine 12 via communications network 16.

A digital copy operation involves both scan and print operationsinitiated locally at the imaging system AP1. Such an operation isdescribed here with reference to the flow chart of a method M1 of theinvention that is practiced in the context of system AP1. Method M1 alsoencompasses the print, scan, and fax operations described above. Forexample, steps S1 to S6 are the same as for a scan operation or afax-transmit operation, while steps P1 to P7 are the same as for a printoperation or a fax-receive operation.

In response to a manual button push, an image-bearing document is fed atscan-related step S1. In a scan operation step S1 is preceded by acommand from a host, while in a fax-transmit operation, step S1 ispreceded by commands from a remote fax.

At step S2, the degree of skew is detected. At step S3, a skew function(e.g., as a skew table) is generated at least in part as a function ofthe detected skew. At step S4, the document is scanned, yieldinguncompensated image data. At step S5, the uncompensated glib image datais buffered; (it may be converted prior to buffering). At step S6, skewcompensation is applied to the buffered data, using the skew functiongenerated at step S3.

After step S6 of the digital copy operation, the skew-compensated datais input to rasterizer 43. In the present case, the skew-compensateddata is input via rasterizer 43, and format-converted as appropriate. Ina scan operation, the skew-compensated data is output to a host computeror network; in a fax-transmit operation, the skew-compensated data(after appropriate conversion to fax format) is transmitted to a remotefax.

Once the scan-skew-compensated data is stored by rasterizer 43, andpreferably before this storage is completed, blank media is fed at stepP1. In a print operation, step P1 can begin in response to a computercommand, while in a fax-receive operation, step P1 can begin in responseto a command from a remote fax. In any case, blank-media skew isdetected at step P2, and used to generate a blank-media skew function(e.g., skew table contents) at step P3.

With appropriate format conversion, scan-skew-compensated image data isbuffered at step P5. Digital print-skew compensation is applied at stepP6 using the skew function generated at step P3. Note that otherfunctions can be implemented in step P6, for example, the image can bescaled (enlarged or reduced). This can happen in response to userselection of scaling factors which are received by controller 47 andused to generate the skew function. The resulting print-skew-compensatedimage is used in generating the actual sheet-media-based copy at printstep P7.

Method M1 provides for an alternative digital copy procedure in whichonly one skew-compensation is required. In this procedure stepsskew-table generation step S3, buffer step S5, and digital-skew step S6are merged into their print-side counterparts. The document skewdetected at step S2 is used in conjunction with blank-media skewdetected at step P2 to generate a skew table at step P3, as indicated bythe dotted line into step P3. The scanned image produced at scan step S4is buffered at step P5. The digital skew at step P6 uses the tablegenerated from both document and blank-media feeds at step P3. Thedocument skew can be stored for use in successive skew tables whenmultiple copies are required.

Additional details of a print operation are explained with reference toFIG. 3. Rasterizer 43 includes dynamic random access memory (DRAM) 91for storing PCL data received from a host computer 10. As the PCL datais converted to raster image data, the later is stored in a1-bit-per-pixel multi-line buffer 93.

Rasterizer 43 then converts the single-bit-per-pixel data according to aresolution-enhancement technology (RET) function 95. RET function 95selects how much toner is applied to an image pixel and how that toneris distributed in the pixel. Toner is applied in multiples of {fraction(1/32)} of the amount required for full pixel coverage. Thus, each pixelis assigned a five-bit value to indicate how many thirty-seconds (of themaximal toner amount for a pixel) are to be applied.

Two additional bits of RET code are used to indicate how the specifiedamount of toner is to be distributed at a pixel. For example, toner canbe centered in a pixel, applied to the left portion of the pixel,applied to the right portion of the pixel, or split between the rightand left portions of the pixel. Two bits of RET image data are used toindicate which of these four distributions are selected. Note that inbuffer 45, the data is shown as a two-dimensional vector. The firstvalue, which assumes values from 0 to 3, indicates the tonerdistribution for a pixel. The second value, which assumes values form 0to 31, indicates the amount of toner.

At any given time, the contents of buffer 45 represent a band of animage to be printed. For a true-fed sheet, each row of buffer 45describes an activation pattern for laser as it generates one rasterline on the sheet media. Each column in buffer 45 corresponds to alongitudinal raster-line pixel position. While buffer 45 is shown forexpository purposes with 5×9 7-bit locations, in practice it is muchlarger, e.g., 120×4800 locations. If the data in buffer 45 is used todrive the laser directly when the sheet media is skewed, the resultingprinted image will be skewed. For the printed image to be true, the datain buffer 45 must be altered in some sense to compensate for thesheet-feed skew.

To a first approximation, skew compensation conceptually involves“moving” pixels (with their toner amounts and distributions) from theirrespective positions in the uncompensated image to correspondinglydifferent positions in the skew-compensated image. In system AP1, thismovement is achieved in two stages. First, a row offset is determined asa function of table column position, which corresponds to position alonga raster line in the uncompensated image. Then a column offset isdetermined as a function of the raster line being generated in theskew-compensated image. In other words, each raster line is begun eithera little sooner or a little later than it would be without the offset.

The skew determined by controller 47 from skew detector 23 (FIG. 1) isused to generate skew table 51, which indicates raster-line (row)offsets as a function of raster position (column). Skew table 51 storesone value per column of buffer 45. Each value corresponds to a rowoffset. Thus, the first value 0 in the leftmost location corresponds fora zero row offset for the leftmost values in buffer 45. Similarly, thevalue 4 in the rightmost location of table 51 corresponds to an offsetof four rows (raster lines) in buffer 45. A diagonal line 97 throughbuffer 45 in FIG. 3 is representative of the skew angle dictated bytable 51.

Fractional entries in table 51 indicate that a weighted average of tworows of a column are to be used determining the amount of toner to beapplied to a pixel in the compensated image. Thus, “2.5” in the sixthlocation of table 51 indicates that the pixels for the sixth pixelposition of the current raster line indicates that the amount of tonerthe corresponding pixel of the compensated image is the evenly-weightedaverage of the pixels in the same column at two-row and three-rowoffsets. If the fractional part is greater than one-half, the greateroffset row is given more weight. While skew table 51 of FIG. 3corresponds to a linear interpolation, the invention provides for otherinterpolation functions, and for more neighboring pixels to be used indetermining the amount of toner to be applied to a pixel in thecompensated image.

The toner distribution for each pixel in the compensated image isdetermined from the toner distributions for the same two pixels in theuncompensated image used to determine toner amounts. In the followingtable, the toner distribution for a pixel in the compensated image isdetermined by finding the intersection of the row corresponding to thetoner distribution from one uncompensated pixel and the columncorresponding to the toner distribution for the other uncompensatedpixel.

TABLE Justification Function for Compensated Image PW Codes 0-Split1-Left 2-Right 3 Center 0-Split 0-Split 1-Left 2-Right 3-Center 1-Left1-Left 1-Left 0-Split 1-Left 2-Right 2-Right 0-Split 2-Right 2-Right3-Center 3-Center 1-Left 2-Right 3-Center

In other embodiments, the amount of toner for a pixel in the compensatedimage is a function of both the amounts and distributions in thepre-compensated image. Likewise, the distribution of toner in a pixel inthe compensated image can be a function of both the distributions andamounts of pixels in the pre-compensated image.

Controller 47 uses table 51 to determine which pixels in theuncompensated image are to be used in determining the values for eachpixel in the compensated image. The needed values are accessed frombuffer 43 and provided to digital skew processor 53. Skew processor 53uses table 51 to determine how the values for the compensated pixel areto be calculated from the data provided to it.

The resulting compensated data is used to drive pulse-width modulator 99of printer device 25. The pulse widths determine the action of the laserof the laser printer media output portion of printer device 25 toachieve the desired toner amount and distribution for each pixel in theskew-compensated image.

Table 51 stores two values in addition to the row offset values. Beforeeach raster line is written, the laser beam is detected two inches fromthe left margin of the sheet. In the event of a true feed, the leftmargin of the sheet is 2″ (1200 pixels) from the detection point. In theevent of skew, the left margin generally has some other value. SystemAP1 “predicts” a beginning left margin from the detected skew. In theexample, the beginning left margin is 1243 pixels. Since the sheet isskewed, the left margin changes for each raster line. Since the printresolution is the same in both the feed and transverse directions, thecolumn—column offset from one raster line to the next is the same as therow offset from one column to the next. Hence, the column-olumn offsetis 0.500. The column—column offset is calculated to greater precisionthan the row offset to minimize errors that accumulate over the lengthof a page.

Alternative embodiments of the invention provide one or more of thefunctions described above. For example, there are print only, scan only,and fax only, embodiments. Different maximum media sizes are providedfor. Larger media, e.g., tabloid, and smaller media, e.g., photo printsized, are provided for. The media can be paper, card stock, envelopes,photo-paper, transparencies, labels, etc.

Different media-feeding arrangements are provided for. For example, acassette can be used for feeding fixed-size media. If this is the onlyblank media source, two fixed edge detectors can be used. If there areboth fixed-size and variable-size media sources, then two fixed edgedetectors plus a movable edge detector can be used. Alternatively,several fixed edge detectors can be used to accommodate various mediawidths and to enhance skew-detection precision. Media feeders have twomovable guides can use two movable edge detectors. In general, the skewdetector can either provide skew measurements or data from which skewdeterminations can be made (e.g., by a controller).

In the preferred embodiment, the rasterizer implements resolutionenhancement technology. More or less sophisticatedresolution-enhancement technology can be used. In addition, colordiffusion can be implemented in various ways in the context of colorprinting and scanning.

In the case of color operations, the skew operation can also be used toassist color registration. For example, in some in-line color laserprinters, four (magenta, cyan, yellow, and black) monochrome colorimages are applied to media or to a transfer belt with registrationmarks during a calibration run. The registration marks can be read forboth displacement and skew. The skew can be used to generateskew-compensations between colors. The displacements can be handled inthe same tables or separately. This approach allows mechanicalconstraints to be relaxed in the design of in-line color lasers.

The present invention has industrial applicability to computers, imageprocessing, and communications generally. In general, the inventionprovides for sophisticated image processing for a very small marginalcost that is more than offset by relaxed mechanical tolerances andextended useful device lifetimes. Printers, scanners, fax machines,digital copiers, etc. can make use of the invention. These and othermodifications to and variations upon the disclosed embodiments areprovided for by the present invention, the scope of which is defined bythe following claims.

1. An image-transfer system comprising: an image-transfer device forconverting between a digital image and a hard-copy media image; amedia-feeder for feeding media to said image-transfer device; a skewdetector for detecting sheet-feed skew in said media; a memory forstoring said digital image; and a controller for applying digital skewcompensation to said digital image as a function of sheet-feed skewdetected by said skew detector, said function indicates raster lineoffsets as a function of raster position, fractional raster-line offsetsindicating interpolation weights for neighboring pixels.
 2. Animage-transfer system comprising: an image-transfer device forconverting between a digital image and a hard-copy media image; amedia-feeder for feeding media to said image-transfer device; a skewdetector for detecting sheet-feed skew in said media; a memory forstoring said digital image, said memory, at any given time, holding lessthan half the data associated with said digital image; and a controllerfor applying digital skew compensation to said digital image as afunction of sheet-feed skew detected by said skew detector.
 3. A systemas recited in claim 2 wherein said digital image data is transferredfrom said image-transfer device to said memory.
 4. A system as recitedin claim 2 wherein said compensated digital image data is transferred tosaid image-transfer device.
 5. A media transfer method comprising thesteps of: feeding sheet media to a image-transfer device; detectingmedia skew in said media as it is fed to said image-transfer device;transferring between a hard-copy image and a digital image stored indigital memory; and digitally skewing said digital image as a functionof said media skew, said function indicating raster line offsets as afunction of raster position, fractional raster-line offsets indicatinginterpolation weights for neighboring pixels.
 6. A media transfer methodcomprising: feeding sheet media to an image-transfer device; detectingmedia skew in said media as it is fed to said image-transfer device;transferring between a hard-copy image and a digital image stored indigital memory so that less than half of said digital image is stored insaid digital memory at any given time; and digitally skewing saiddigital image as a function of said media skew.
 7. A method as recitedin claim 6 wherein said digitally skewing step occurs after saidtransferring step.
 8. A method as recited in claim 6 wherein saiddigitally skewing step occurs before said transferring step.